The present invention relates to a hydraulically operated continuously variable transmission including a hydraulic pump and a hydraulic motor which are interconnected by a closed hydraulic circuit, and more particularly to a distribution ring for distributing oil discharged from a hydraulic pump to a hydraulic motor in a hydraulically operated continuously variable transmission.
There have been proposed various hydraulically operated continuously variable transmissions for use in automobiles, including a swash-plate axial-plunger hydraulic pump having a pump cylinder coupled to an input shaft and a plurality of pump plungers slidably disposed in respective cylinder holes defined in the pump cylinder in an annular pattern around an axis of rotation of the pump cylinder, the hydraulic pump having an outlet port, a swash-plate axial-plunger hydraulic motor having a motor cylinder coupled to an output shaft and a plurality of motor plungers slidably disposed in respective cylinder holes defined in the motor cylinder in an annular pattern around an axis of rotation of the motor cylinder, the hydraulic motor having an inlet port, and a closed hydraulic circuit interconnecting the hydraulic pump and the hydraulic motor. Power is transmitted by the transmission through relative rotation between the motor cylinder and the pump cylinder.
As disclosed in Japanese Patent Publications Nos. 32-7159 and 41-3208, for example, communication ports arranged at substantially equal angular intervals in communication with the respective cylinder holes in the motor cylinder are defined in the motor cylinder and open at an axial end face thereof in an annular pattern around the axis of rotation of the motor cylinder. An annular distribution ring is disposed eccentrically with respect to the axis of rotation of the motor cylinder and held in slidable contact with the end face of the motor cylinder. Oil discharged from the hydraulic pump is introduced into the distribution ring, whereas oil discharged from the motor cylinder is introduced around the distribution ring.
The outlet and inlet ports of the pump are brought into successive communication with the communication ports in the motor cylinder in response to relative rotation between the distribution ring and the motor cylinder for reciprocally moving the annularly arranged motor plungers.
In the above structure, the communication ports are divided into a group of communication ports in an expansion stroke and a group of communication ports in a contraction stroke by the annular distribution ring which is disposed eccentrically with respect to the center of the pitch circle of the communication ports. Therefore, it would be difficult to provide a large open area over a wide angle in the direction of rotation. Particularly when the hydraulic motor rotates at a high speed, the resistance to suction is excessively large in a suction stroke, thus generating cavitation, and the resistance to discharge is excessively large in a discharge stroke, thus producing a pressure loss. At any rate, the conventional arrangement is responsible for lowering the efficiency of operation of the hydraulic motor.